1. Field of the Invention
The invention relates to a microphone amplifier circuit for a capacitor microphone.
2. Description of the Related Art
A MEMS microphone and an electret capacitor microphone (ECM) are known as a type of capacitor microphone.
The MEMS microphone is a capacitor including two electrode plates called a diaphragm and a back plate which face close to each other, and such a structure is formed on a silicon substrate by a MEMS (Micro Electro Mechanical Systems) technique. The MEMS microphone is resistant to the temperature of a standard solder reflow process, and is soldered on a printed board together with other components, for example.
In order to operate the MEMS microphone, a relatively high direct bias voltage is necessary. By applying this bias voltage, constant electric charge Q is charged in the capacitor forming the MEMS microphone. In this state, when the diaphragm vibrates by sound pressure, the electrostatic capacitance C of the capacitor changes to change voltage V between the terminals. The change of the voltage V is outputted as an audio signal.
On the other hand, the ECM uses an electret element which holds electric charge semipermanently and does not need a bias voltage.
A microphone amplifier circuit for a capacitor microphone is described in the Japanese Patent Application Publication Nos. 2010-245729, 2008-153981 and 2001-102875.
FIGS. 5A and 5B are conceptual diagrams showing the SNR (signal-noise ratio) characteristic of a conventional microphone amplifier circuit 30. FIG. 5A shows a case where the input conversion noise A of the microphone amplifier circuit 30 is larger than the noise M of a capacitor microphone 10 such as an ECM. In this case, the input conversion noise A is dominant as a noise source, and the SNR is the ratio of the audio signal S of the capacitor microphone 10 to the input conversion noise A which are amplified by the microphone amplifier circuit 30.
FIG. 5B shows a case where the input conversion noise A of the microphone amplifier circuit 30 is reduced to less than the noise M of the capacitor microphone 10. In this case, the noise M of the capacitor microphone 10 is dominant as a noise source, and the SNR is the ratio of the audio signal S of the microphone element 10 to the noise M of the capacitor microphone 10 which are amplified by the microphone amplifier circuit 30.
In other words, the noise of the microphone amplifier circuit 30 is determined by the larger of the input conversion noise A and the noise M of the capacitor microphone 10. Then, even if the input conversion noise A of the microphone amplifier circuit 30 is reduced as much as possible, the enhancement of the SNR is limited by the noise M of the capacitor microphone 10.
The invention is to provide a microphone amplifier circuit which enhances the SNR and expands the dynamic range of the microphone amplifier circuit.